Filtration methods and apparatus are described in numerous sources. The final stage of water purification at most of the existing water purification plants is so-called rapid filtration, also known as deep bed filtration, or depth filtration. Deep bed filtration methods have also been modified and adopted for the tertiary treatment of wastewater. Metcalf & Eddy, Wastewater Engineering, Treatment and Reuse, Fourth Edition, Revised by George Tchobanoglous, Franklin L. Burton, and H. David Stensel, McGraw Hill Book Company, 2003, provides a review of these methods.
Besides the removal of suspended solids and turbidity in the entire depth of the bed, these beds act as contact reactors for chemical, physical chemical and biological processes, for example: contact coagulation and improved suspended solids removal, removal of phosphorus with the addition of aluminum salts, BOD reduction due to the attached growth biological processes. The ability of the deep bed filters to perform as contact reactors is a significant advantage.
Membrane filters present a very efficient barrier for removal of suspended solids and turbidity, and additionally remove bacteria and even viruses. The latter is essentially a unique capability that can hardly be met by other means at this time. Membranes can be operated in low to high concentration suspensions.
Fine media, in deep bed filtration removes suspended solids and turbidity well, but the “dirt capacity” of fine media is low thus requiring frequent back washes and, therefore, large quantity of backwash water. Coarser media has increased “dirt capacity” and less frequent backwashes, but the treatment efficiency is lower. The use of coarse and fine media (dual, multiple, mixed, matrix media) can increase the “dirt capacity” as compared to fine media and provide the required suspended solids removal, but at a considerable increase in the system and its operation complexities.
A reasonable balance between treatment efficiency, frequency, intensity, and duration of backwash and the length and the rate of the filtration portion of the cycle can require a considerable volume of either fine, or coarse, or dual (multiple and other) media beds.
Deep bed filters with downflow through single (stratified or mixed), multiple (e.g. anthracite, sand, garnet, or ilmenite), matrix or carcass (about 2 m deep approximately 50 mm stone bed filled for about half depth by sand) media, with upflow through stratified granular media heavier than water, cross-flow (horizontal flow) through vertically uniform deep bed media, dual-flux, respectively, from the top and from the bottom across the upper and bottom portions of the deep bed with filtrate collection means between the top and the bottom portions of the bed, continuous filtration with moving deep bed, and continuous filtration with periodic regeneration of the “spent” portion of the deep bed.
Most deep bed filters are operated semi-continuously with intermittent filtration and backwash periods. The flow in the filter can be classified as upflow, downflow, or crossflow. Two modes of operation, the declining or constant rate of filtration can be used, both modes present operational difficulties, particularly with the physical balancing of the total influent and effluent flows, and controlling the filtration rates in individual filters and among multiple filters.
Deep bed media require thorough backwash followed by a period of wasting the “first” filtrate having increased suspended solids and turbidity. The wasting of the first filtrate increases the percentage of water spent for the deep bed filter operation reducing the filter capacity. During the backwash, the period of filtration is interrupted, thus decreasing the capacity of filters. The backwash must be done using filtrate that further reduces the effective capacity of the deep bed filters.
If the suspended solids concentration in the raw water source is less than about 150 mg/l contact filtration can be used. Daniel Mints, an inventor of contact filtration, recommended the value of 150 mg/l. In practice, the actual values are typically less than 150 mg/l. At greater suspended solids concentrations, steps of coagulation, flocculation, and settling need to be conducted prior to filtration.
Systems for raw water distribution-filtrate collection, as well as for backwash water, and/or air distribution and spent backwash water collection are complex and expensive, mainly, due to the need of uniformly distributing the raw water and backwash water (and air) flows and collecting the filtrate and spent backwash water. Difficulties with controlling the uniformity of flows result in the size limitation for the deep bed filters, usually up to 6×6 m2. Larger filters may be used at very large treatment plants, but often with compromised efficiency and/or large cost increase of water distribution-collection systems.
Membrane filtration for water purification and wastewater treatment has been more recently developed and applied. As the cost of membranes decreases, the technology is becoming affordable and is gaining popularity. Membrane filtration is used for treatment of low-to-highly concentrated suspensions and can produce virtually suspended solids and turbidity free effluents. For example, membranes are used in membrane bioreactors for biological treatment of wastewater with the concentration of mixed liquor suspended solids approximately 10 g/l. Plugging of membranes often occurs in such systems. The difficulty and the cost of membrane restoration is a disadvantage of such systems. Sometimes, conventional bioreactors with clarifiers are followed by membrane filtration. This reduces the required membrane surface and the capital and operating costs of membrane units, however, the bioreactor volume is not reduced. Sometimes, membranes are installed after deep bed filters, thus the periods between membrane regenerations are further increased. In the known membrane systems, admixtures to the water concentrate in a thin surface layer and often form mineral, organic, and biological deposits, for example, such as hard precipitates of hardness salts and metal phosphates, gels of metal hydroxides, and biological slimes. These deposits accumulate at, adhere to, and plug the pores on the membrane surface, thus reducing the filtration flux across the membrane. In some membrane systems, backpulses of filtrate and air scour are used for in-situ (continuous or very frequent) cleaning of the membrane surface. However, periodic chemical surface cleaning is also required; such cleaning is often done outside the main filtration system.
In contrast to deep bed filters, membranes do not provide reaction volume (deep bed) for removing many constituents of admixtures, and accumulation volume for the reactants and the products, including deposits. Accordingly, many objectives of water purification or tertiary treatment of wastewater cannot be achieved as effectively as in deep beds (e.g., BOD and COD reduction, phosphorus removal with metal salts, heavy metals removal in form of precipitates within the deep bed, and similar processes, in respect to the need in reaction volume and in retention means and contact media).
Membrane media concentrates many constituents of admixture in a thin layer at the surface where they can form deposits by themselves or by reacting with each other. Most common admixtures are (1) mineral precipitates forming gels like metal hydroxides, or hard precipitates in form of insoluble or poorly soluble metal carbonates and phosphates, (2) organic admixtures, for example, oily and fatty materials, and (3) biological slimes. These deposits plug membrane pores and reduce the filtration flux.
There are two modes of membrane cleaning: (1) on-going, usually including continuous or frequent air scour and periodic back pulses of filtrate (for hollow fiber media), and (2) periodic chemical treatment and restoration of membranes. These are complex procedures. Chemical treatment may shorten the life of membranes. In either case, the filtration capacity of membranes exposed to concentrated sticky deposits declines over time more rapidly than in cleaner environment.